1. Field of the Invention
The present invention relates to a predistortion type nonlinear distortion compensating circuit (predistorter) which compensates for nonlinear distortion by multiplying an input signal by the inverse characteristic of the nonlinear characteristic of an AMP in, e.g., a baseband unit of a transmitter used in a CDMA communication system or the like.
2. Description of the Prior Art
In recent digital mobile communication, a CDMA communication system is beginning to be used in many cases in order to increase the anti-interference capability. In this CDMA communication system, the instantaneous power is higher than the mean power. Therefore, to reduce the adjacent-channel leakage power, it is necessary to hold the linearity of a high-output power amplifier on the transmitting side up to a very high output level. Since this increases the size of the output amplifier, the amplifier becomes expensive and consumes high power. To implement a high-efficiency, low-distortion transmitting unit, therefore, a predistortion technique is extensively studied to make a nonlinear output amplifier usable by applying the nonlinear inverse characteristic in a baseband unit before the transmitting unit. This technique is excellent in stability and space saving properties because distortion can be compensated by digital signal processing in the baseband unit.
Techniques pertaining to this predistortion technique are also disclosed in, e.g., Japanese Unexamined Patent Publication No. 2000-31869 (a transmitting apparatus and method having a predistortion compensating function), Japanese Unexamined Patent Publication No. 2001-268149 (an adaptive predistortion compensating apparatus and adaptive predistortion compensating method), and Japanese Unexamined Patent Publication No. 2001-268150 (a linearizer).
In a transmitting apparatus including a nonlinear distortion compensating circuit of this type, the nonlinear inverse characteristic is added to an input signal by a predistorter. After that, the input signal is quadrature-modulated, amplified by an AMP, and output. Generally, the nonlinear characteristic of the AMP is estimated by comparing an input to the predistorter with a signal obtained by quadrature-modulating the AMP output, and the inverse characteristic of this nonlinear characteristic is set in a memory of the predistorter.
FIG. 1 is a block diagram showing the arrangement of the main parts of a transmitting apparatus including a conventional example of a nonlinear distortion compensating circuit (predistorter) 200 as the core of the predistortion technique.
A transmitting apparatus 100 shown in FIG. 1 includes a nonlinear distortion compensating circuit main unit 300, two D/A (Digital/Analog) converters 101a and 101b, local oscillator 102, quadrature modulator 103, power amplifier 104, antenna 105, and the like. The nonlinear distortion compensating circuit main unit 300 includes, e.g., two input terminals, i.e., Ti (I component input terminal) and Tq (Q component input terminal), for a transmission IQ signal (in which an in-phase component and quadrature component are expressed by complex numbers), a complex multiplier 10, an amplitude converter 20, and a memory 40. The D/A converters 101a and 101b convert digital signals Si and Sq to be transmitted, which are output from the nonlinear distortion compensating circuit main unit 300, into analog signals. The quadrature modulator 103 quadrature-modulates the outputs from the two D/A converters by using an oscillation signal from the local oscillator. The power amplifier 104 amplifies the power of the output from the quadrature modulator 103. The antenna 105 transmits the output from the power amplifier 104 as an electric wave.
A coupler 201 for extracting the output from the power amplifier 104 is inserted between the power amplifier 104 and the antenna 105. An attenuator 202, a quadrature demodulator 203, and a pair of A/D (Analog/Digital) converters 204a and 204b are connected in series with the coupled output from the coupler 201 to form a feedback loop of the output signal from the power amplifier 104. The outputs from the two A/D converters are input to a memory correction arithmetic unit 30. The output from the memory correction arithmetic unit 30 is connected to the memory 40 of the nonlinear distortion compensating circuit main unit.
The memory correction arithmetic unit 30 includes a distortion compensation value calculator 31. On the basis of an input signal (transmission IQ signal) to the transmitting apparatus 100 and a distortion compensation value corresponding to the amplitudes and phases of the outputs (feedback signals) from the A/D converters 204a and 204b, the distortion compensation value calculator 31 calculates an updated value of the distortion compensation value to be stored in the memory 40.
The nonlinear distortion compensating circuit main unit 300 comprises the complex multiplier 10 inserted between the terminals Ti and Tq and the two D/A converters 101a and 101b, the amplitude converter 20 for calculating an amplitude X of input signals from the input terminals Ti and Tq, and the memory 40 which stores a distortion compensation value corresponding to the amplitude X and supplies this distortion compensation value to the complex multiplier 10. The memory 40 includes an address generator 41 and compensation data table (memory) 42. These components are controlled by a distortion compensation controller (not shown). The nonlinear distortion compensating circuit main unit 300 is formed by, e.g., a well-known microcomputer circuit which includes a ROM (not shown) storing programs and the like and a RAM (not shown).
In the memory 40, the compensation data table 42 storing compensation values each related to the amplitude X of a predicted input value so as to be used in distortion compensation is stored on a programmable memory. Assume that the amplitude X of an input signal is expressed by normalization so as not to exceed a maximum of 1. In the compensation data table 42, a plurality of compensation data by which the inverse characteristic of a predetermined nonlinear characteristic of the power amplifier 104 is expressed by complex numbers with respect to input amplitudes are set as initial values. Alternatively, it is also possible to unconditionally set each compensation data like, e.g., Si=1 and Sq=0, and gradually cause the data table to learn.
A nonlinear distortion compensating operation by this transmitting apparatus will be explained below. In the nonlinear distortion compensating circuit main unit, the amplitude converter 20 calculates the amplitude (the absolute value of a complex number) of a transmission IQ signal (in which an in-phase component and quadrature component are expressed by complex numbers) input from a signal source in the input stage to the input terminals Ti and Tq, and outputs the calculated amplitude to the memory 40. The address generator 41 of the memory 40 outputs a corresponding address in the compensation data table 42. The compensation data table 42 stores compensation data by which the inverse characteristic of the nonlinear characteristic of the amplifier is expressed by complex numbers with respect to input amplitudes. Compensation data (the nonlinear inverse characteristic) in the designated address is output from the memory 40 to the complex multiplier 10.
The complex multiplier 10 multiplies the transmission IQ signal by the compensation data output in a complex form, thereby adding the nonlinear inverse characteristic to the input transmission IQ signal. This distortion-compensated signal is output to the power amplifier 104 via the D/A converters 101a and 101b and quadrature modulator 103. The output from the power amplifier 104 is output from the antenna 105 via the coupler 201.
At the same time, the transmission signal detected and extracted by the coupler 201 connected to the output of the power amplifier 104 is fed back, as is well known, through the feedback loop made up of the attenuator 202, quadrature modulator 203, and D/A converters 204a and 204b. That is, a portion of the transmission signal is detected by the coupler 201 and attenuated by the attenuator 202. The quadrature modulator 203 detects an IQ signal (to be referred to as a detected IQ signal hereinafter), and this signal is converted into a digital signal by the A/D converters 204a and 204b. The digital signal is input to the memory correction arithmetic unit 30. The distortion compensation value calculator 31 detects an error between this digital signal and the transmission IQ signal, and the corresponding distortion compensation value in the compensation data table 42 of the memory 40 is updated.
That is, the memory correction arithmetic unit 30 replaces the distortion compensation value of a record with the updated distortion compensation value obtained from the memory correction arithmetic unit 30. An appropriate operation can be performed by thus updating the distortion compensation data table 42 as needed. Note that a certain time is necessary before the distortion compensation data table 31 is set to an appropriate value after the operation is started.
When the result of conversion of an input signal by the amplitude converter is to be output as N-bit digital data in the above system, a capacity of 2N words is necessary as the capacity of the distortion compensation data table (memory) 42 for storing the nonlinear inverse characteristic. Since the memory capacity has a large effect on the circuit scale, it is important to reduce the memory capacity.